Fuel cell assemblies employing proton exchange membranes are well known. Such assemblies typically comprise a stack of individual fuel cells, each fuel cell having an anode and a cathode separated by a catalytic proton exchange membrane (PEM). The fuel cells in the stack are connected in series electrically to provide a desired voltage output. Gaseous fuel, in the form of hydrogen or hydrogen-containing mixtures such as “reformed” hydrocarbons, flows adjacent to a first side of the membrane, and oxygen, typically in the form of air, flows adjacent to the opposite side of the membrane. Hydrogen is catalytically oxidized at the anode-membrane interface, and the resulting proton, H+, migrates through the membrane to the cathode-membrane interface where it combines with anionic oxygen, O31 2, to form water. Protons migrate only in those areas of the fuel cell in which the anode and cathode are directly opposed across the membrane. Electrons flow from the anode through an external circuit to the cathode, doing electrical work in a load in the circuit.
A complete fuel cell assembly typically comprises a plurality of individual fuel cells connected in series to form one or more fuel cell stacks. In a preferred embodiment, a bipolar plate assembly, comprising an anode, a cathode, and having formed passages for the flow of hydrogen to the anode and air to the cathode, is disposed adjacent an element known in the art as a Membrane Electrode Assembly (MEA). A repeating pattern of alternating bipolar plate assemblies and MEA elements form a stacked fuel cell assembly.
Preferably, a Gas Diffusion Layer (GDL) element is also included between each bipolar plate assembly and an adjacent MEA to promote the distribution of gas uniformly over both the anode and the cathode.
At the outer edges of the stacked fuel cell assembly, the bipolar plate assemblies and MEA elements are sealed together by gasket elements to contain the reactant gases and/or coolant within the assembly. Thus, an important aspect of forming a stacked fuel cell assembly is preventing leakage between the plate assemblies.
Another important consideration is precisely aligning the multitude of bipolar plate assemblies. In the prior art, a fuel cell stack typically is formed by assembling, one at a time, alternating bipolar plate assemblies and MEA elements to form a fuel cell unit. A full stack for some applications comprising about 60 individual fuel cell units, and for some other applications up to 200 units. Typically, the bipolar plate assemblies and MEAs are bonded along their outer edges with silicone rubber or other inert, curable sealant, making any subsequent disassembly difficult, time-consuming, and hazardous to the individual stack elements.
It is known to provide alignment holes in the stack and to use an assembly fixture having alignment pins. A problem arises in this arrangement however, in that the assembly cannot be tested for perimeter leaks until all the elements have been assembled together and the sealant cured. If a leak is detected, the stack must be disassembled down to the point of the leak to fix the leak. Once a stack has passed the leak test, it is performance tested. Again, if a bipolar plate assembly or MEA is found defective, the stack must be disassembled and reworked.
What is needed is means for intermediate testing during assembly of a fuel cell stack to limit the amount of reworking necessary when any defect is found.
It is a principal object of the present invention to reduce rework labor in assembling a fuel cell stack.
It is a further object of the present invention to reduce the cost of manufacturing a fuel cell assembly.